With the development of mobile communication technology and the evolution of communication networks, various types of mobile communication systems have been developed. In order to resolve the problem of global roaming between the mobile communication systems, International Mobile Telecommunications (IMT)-2000 systems have been developed. The IMT-2000 systems are divided into a CDMA 2000-based synchronous system and a WCDMA-based asynchronous system.
Furthermore, in order to support global roaming between mobile communication systems, Dual Band Dual Mode (DBDM) mobile communication terminals usable both in a synchronous system and in an asynchronous system have been developed. Using such a mobile communication terminal, it is possible to use different types of services both in the area of an asynchronous system and the area of a synchronous system.
Currently, an asynchronous mobile communication system is being constructed in areas in which high demand for service exists and, therefore, a synchronous mobile communication system is being evolved into a form in which the service area thereof includes the service areas of the asynchronous mobile communication system. When a user moves between the asynchronous mobile communication system and the synchronous mobile communication system, handover between the systems is required in order to provide continuous service.
Particularly, when the mobile communication terminal is located on the border between an asynchronous area and a synchronous area, or a shadow area in the communication, such as underground, precise handover for the mobile communication terminal must be performed.
FIG. 1 is a conceptual diagram illustrating handover in a mobile communication network in which an asynchronous network and a synchronous network coexist.
As illustrated, in the case in which a mobile communication terminal 10 which is using service in an asynchronous area A moves into the overlap area of the asynchronous area A and a synchronous area B (through the movement path C of the terminal), there are problems in that the intensity of signals between the asynchronous area A and the mobile communication terminal 10 is similar to that of signals between the synchronous area B and the mobile communication terminal 10, and variation in electric field strength is considerable, so that handover is not precisely performed and it takes a long time to perform the handover.